The present invention concerns a method and device for decoding a coded digital signal, and more particularly a method and device for decoding a digital signal which was quantized during coding.
The coding of a digital signal is normally used for reducing the transmission time for this signal or to reduce the space required for storing such a digital signal. The decoding makes it possible to reconstitute, in order to use it, a digital signal which was previously coded.
The present invention concerns the decoding of digital signals coded by methods of compressing digital signals with loss, and is particularly related to compression methods in which quantization methods are used.
Quantization techniques make it possible, in a known fashion, to transform a continuous signal into a discrete signal, or to transform one discrete signal into another discrete signal which is less finely represented.
Such a quantization of the digital signal makes it possible to represent the signal by means of a finite number of bits; the larger the quantization step (in scalar quantization) or the smaller the dictionary (in vector quantization), the smaller the number of bits.
It is clear that, the larger the size of the quantization cells in which all the vectors of the signal are coded by the same quantization symbol, the greater the compression of the signal whilst, the smaller the size of the quantization cells, the smaller the distortion appearing on decoding.
Known techniques of dequantizing a coded signal are described in the book entitled "Vector Quantization and Signal Compression" by A. Gersho et R. M. Gray, published by Kluwer Academic Publishers, Boston, Mass., 1992, pages 173-185. These techniques use the centre of the quantization cell as the dequantization vector, or restoration level.